The present invention relates to spinal implants and stabilization systems. More particularly, the invention concerns a dynamic stabilization system that is disposed in between adjacent vertebrae.
In the past, the principal protocol for the treatment of the spine has been rigid fixation combined with fusion of the affected vertebral body or intervertebral disc. Arthrodesis, as this approach is known, has been achieved with a variety of rigid fixation elements, such as spinal rods or plates that are rigidly fixed to a vertebra using bone screws, bone bolts and spinal hooks. However, spinal fusion has been recognized to have limitations in the treatment of disc degeneration, especially in the earlier stages of the degeneration where it may be unnecessary to eliminate motion of the spinal motion segments.
Clinical studies suggest that cells of the intervertebral disc respond favorably to reduced (but not eliminated) mechanical loading through deposition of extracellular matrix proteins (collagen, proteoglycan, fibronectin, etc.) into the disc space. In some cases, a degenerated disc may simply involve a mechanically overloaded and hypermobile segment that can be repaired by reversing the mechanically damaging load environment. For instance, clinical experiences with dynamic stabilization systems suggest that the disc becomes increasingly hydrated over time, as judged by MRI scanning.
Spinal instability is a recognized effect of degenerative disc disease. In contrast to arthrodesis, arthroplasty is a protocol that contemplates restoring segmental spinal motion while treating the degenerative condition. Arthroplasty has been successfully used in the treatment of degenerative conditions of the hip and knee. In recent years, efforts have been made to implement arthroplasty in the spine, and most particularly in the intervertebral space. Intradiscal arthroplasty is now clinically available in the form of articulating prosthetic discs and polymeric disc nucleus replacements. With the availability of viable intradiscal arthroplasty devices, interest has grown in providing some means for dynamic spinal stabilization—i.e., stabilization that still permits some degree of mobility between spinal segments.
Drawing from the approaches developed for intradiscal arthroplasty, efforts have made to develop an extradiscal arthroplasty. These systems offer the advantage of “soft stabilization” that limit, rather than eliminate, spinal segment motion. Current theories suggest that preventing movement of the spinal segments may not be a significant factor in clinical success of spinal stabilization systems. Instead, these theories focus on creating a normal loading pattern for the spine as a primary vehicle for successful spinal instrumentation. Thus, the goals for dynamic stabilization has been to restrict movement of the spine to a zone or range where normal or near normal loading of the spinal segments can occur. At the same time, dynamic stabilization techniques have sought to prevent the spine from adopting a position or orientation where abnormal loading of the spine can occur. An acceptable outcome of dynamic stabilization is to make the instrumented spinal level more flexible than rigid (which would occur with fusion), but less flexible than normal.
One approach to achieve these goals for dynamic stabilization utilizes the spinous process. Thus, in one system, flexible “ligament” are engaged around the spinous process of adjacent vertebrae. Another form of flexible “ligament” is attached to the spinous process by way of small screws. In yet another approach, a polymeric spacer is held in place between the adjacent spinous processes. One system utilizes a coil spring that spans several vertebrae and that is anchored to the lamina of the end vertebrae. In one version, a rod extends through part of the coil spring to control rotation.
A similar construct employs a bearing cushion fixed between the spinous processes of adjacent vertebrae, as depicted in FIG. 1. This cushion 1 includes a U-shaped member having opposite brackets 2 for engagement to the spinous processes SP and separated by a flexible central portion 3. In certain embodiments, a cushion element 4 is disposed between the brackets 2 to limit the movement of the brackets, and therefore the adjacent vertebrae, towards each other. Details of this bearing cushion can be discerned from U.S. Pat. No. 5,645,599, the disclosure of which is incorporated by reference herein. As with the ligament-type systems, this bearing cushion resists movement of the adjacent vertebrae in flexion as well as extension. The addition of the cushion element increases the resistance in extension.
In another approach, resistance to flexion is eliminated, with the damping effect experienced only during extension. In this system, an insert 5 is held to the spinous process of a vertebra by a band or cable 7 encircling the spinous process. The insert 5 includes lips or wings 6 that contact each other when the anterior aspect of the vertebrae move together in extension. Details of this system are found in U.S. Pat. No. 5,011,484, the disclosure of which is incorporated herein by reference.
Some dynamic stabilization systems have relied upon fixation to the pedicle of the vertebrae. In these types of systems, a pedicle screw is threaded into the pedicle of adjacent vertebrae. A member spans between the heads of the pedicle screws to limit the movement of the spinal segments. In one device, known as the Graf Ligament, a non-elastic band is wrapped around pedicle screw anchors. The non-elastic bands lock the spinal segment into lordosis, while permitting minimal rotation movements of the spine.
Another system utilizing pedicle screws, provided by Zimmer, Inc. as the Dynesys System, incorporates a polymeric cylinder between the bone anchors. The Dynesys System permits, but limits, relative motion between adjacent vertebrae. An apparatus known as the FASS System essentially integrates features from the Graf and Dynesys systems.
The DSS System employs still another approach by including a spring element connected to pedicle screws. The spring element is contained within a polyurethane tube to prevent tissue ingrowth. Finally, some systems utilize a rigid member, such as a spinal plate, spanning between vertebrae. The flexible stabilization feature is incorporated into the interface between the pedicle screw and the rigid member, such as through a flexible washer or a spherical screw-plate interface.
There remains a need for a dynamic stabilization system that efficiently permits dynamic movement of the spinal motion segments while restricting the range of this movement.